The objective of this proposal is to determine the role of prolactin in human breast cancer. Major emphasis will be placed on examining the effect of prolactin on normal breast tissue from humans and primates and comparing the effects with those observed in studying cancer tissue. In vivo studies on the effects of prolactin and other hormones in normal physiologic states and in disease states will be evaluated as well. The effects of prolactin will be judged by radio-immunoassay measurements of prolactin and milk proteins including alpha-lactalbumin, casein and lactoferrin. Prolactin receptor studies will be evaluated as well. Patients with prolactin-sensitive breast cancers will be identified and hopefully prolactin-inhibiting drugs might be of value in the treatment of their disease. It is likely that these studies will provide an in depth look at prolactin in both normal and abnormal phsyiologic states.